Essentially, a solar cell is a semiconductor device having a diode structure which includes one p-n junction therein. Such a solar cell generates a relatively low voltage. Accordingly, when a practical solar generator is desired, a plurality of solar cells are usually connected in series so that the sum of the individually generated voltages becomes equal to a desired voltage. It is also well-known that a plurality of such series combinations of solar cells can be connected in parallel to provide a desired output current.
When some of a plurality of serially connected cells of a solar generator having the above-described structure enter into a shadow during operation so that no solar radiation is incident on those cells, the cells in the shadow cannot generate voltages but, rather, block current flow in the solar generator. Therefore the power provided by the solar generator device considerably decreases. In addition, when cells are considered in terms of diodes, the p-n junction of a cell in the shadow receives the sum of voltages generated by other cells connected in series, as a reverse voltage across it. If the reverse breakdown voltage and current carrying capacity is small, the p-n junction may be damaged.
There are two methods for eliminating the above-described problem. A first one is to increase the reverse breakdown voltage of a cell by employing a low impurity concentration base layer of the cell. Generally, however, it is required that a p-n junction of a solar cell be near to the solar radiation incident surface of the cell and, in particular, the distance from the surface of a p-n junction of a cell for extraterrestrial use should be less than 0.3-0.5 .mu.m in order to increase the response to short wave length radiation. Practically, it is very difficult to form such a shallow p-n junction by a diffusion technique in a base layer which has an impurity concentration of, for example, 10.sup.13 -10.sup.14 atoms/cm.sup.3 that is required for providing a reverse breakdown voltage of several hundred volts. Also, it is difficult to obtain such a low impurity concentration by a crystal growing technique. Thus, generally speaking, there is a limit on the reverse breakdown voltage which can be increased by this first method. In particular, this method is not suitable for high voltage generators.
A second method is to connect a protective diode in parallel in reverse polarity with a cell. This method of using a reverse polarity, parallel-connected diode may be practical, but, in order to connect protective diodes for a number of series combinations of cell, not only is space for the protective diodes required, but the fabrication of solar generator devices having such protective diodes takes time and labor, which causes the devices to be expensive. Furthermore, the reliability of such devices decreases due to the increase in number of constituent components. This is a great disadvantage, particularly, in devices for extraterrestrial use and, therefore, require high reliability.
U.S. Pat. No. 3,912,539 issued to Vincent Magee on Oct. 14, 1975 and assigned to Ferranti Limited discloses a solar cell array comprising a plurality of cells. Each of the cells of this U.S. patent comprises a photovoltaic section of a larger area and a diode section of a smaller area. The photovoltaic sections are connected in series with each other, and the diode sections are connected, as protective diodes, in parallel with the respective photovoltaic sections associated therewith. The array of this type is relatively free of the above-stated disadvantages. However, these cells are formed by separating a single p-n junction into a larger area photovoltaic section and a smaller area diode section with a groove, and the thus formed photovoltaic and diode sections share one of either the p and n layers. Accordingly, in a cell array, the diode section of each cell is connected in parallel with the photovoltaic section of the next cell. This requires complicated and time-consuming connections in the array. In addition, it is disadvantageously necessary to connect a discrete protective diode for the photovoltaic section of the first stage cell.
H. Matsutani discloses, in Japanese Patent Application No. SHO 59-190041 (Laid-Open Patent Publication No. SHO 61-67968), a cell which comprises a p-n junction forming a photovoltaic section, and a small opposite conductivity type diffusion region in a portion of the p-type or n-type layer of the p-n junction of the photovoltaic section. The small diffusion layer forms another p-n junction with the p-type or n-type layer in which it is formed. This additional p-n junction functions as a protective diode. The cells disclosed in this Japanese application, however, also have the same disadvantages as those of the cells disclosed in the aforementioned U.S. patent, because the photovoltaic section and the protective diode share a p-type or n-type layer.
Then, it has been proposed to form a photovoltaic section and a protective diode which are integral with each other and connected in an inverse-parallel relation, so that a solar generator can be produced without using discrete diodes as protective diodes.
GaAs solar cells based on this concept are disclosed by M. Yoshida et al in Japanese Patent Application No. SHO 56-69607 (Laid-Open Patent Publication No. SHO 57-184225) and Japanese Patent Application No. SHO 56-90109 (Laid-Open Patent Publication No. SHO 57-204180).
In order to form a built-in protective diode in a cell, it is usually necessary to electrically isolate an n-type (or p-type) layer of a photovoltaic section and an n-type (or p-type) layer of the protective diode section from each other. According to the techniques disclosed in the last two Japanese patent applications, in order to provide this electrical isolation, two spaced-apart solar cell structures (i.e. p-n junction elements) are formed on a semi-insulating substrate of a material such as GaAs and one of the two solar cell structures is used as a photovoltaic section and the other as a protective diode.
Since the substrate is semi-insulating in a solar cell of the type disclosed in the above-mentioned Japanese Published Patent Applications No. SHO 57-184255 and No. SHO 57-204180 which comprises a photovoltaic section and a protective diode, an n-type (or p-type) electrode that is a counterpart of the electrode on the light-incident surface of the photovoltaic section cannot be formed on the rear surface of the substrate. In order to form the n-type (or p-type) electrode, a portion of a p-type (or n-type) layer of the photovoltaic portion adjacent to the light-incident surface is removed to expose an n-type (or p-type) layer beneath the removed portion of the p-type (or n-type) layer, and then the n-type (or p-type) electrode can be formed on the exposed n-type (or p-type) layer portion. In this structure, however, because of the lateral resistance of the n-type (or p-type) layer, the internal series resistance of the solar generator increases and, therefore, the conversion efficiency decreases. Furthermore, since a portion of the light-incident surface is occupied by the n-type (or p-type) electrode, the effective light receiving area is reduced, which further decreases the conversion efficiency. In order to expose the portion of the n-type (or p-type) layer where the electrode is to be formed, the depth of etching has to be precisely controlled when the p-type (or n-type) layer overlying the portion of the n-type (or p-type) layer is selectively etched. If the p-type (or n-type) and the n-type (or p-type) layers are of the same material, such a precise etching depth control is very difficult.
An object of the present invention is to provide a solar cell comprising an integral combination of a photovoltaic section and a protective diode which are connected in an inverse-parallel relation, and, more particularly, to such a solar cell suitable for being connected in series with a plurality of other like cells to form a solar generator device which can produce a desired magnitude of output voltage.
Another object of the present invention is to provide a solar cell comprising an integral combination of a photovoltaic section and a protective diode which are connected in an inverse-parallel relation, and, more particularly, to such a solar cell which has small internal series resistance and a relatively large effective light receiving area. According to this feature of the invention, a practical solution to the problem of reduction in conversion efficiency is given.
Still another object of the present invention is to provide such a solar cell as stated above, of which fabrication does not require precise control of processing conditions that have been difficult, but only requires relatively simple processing.
A further object of the present invention is to provide a relatively simple method of fabricating a novel solar cell comprising an integral combination of a photovoltaic section and a protective diode connected in an inverse-parallel relation.